Method of avoiding lapped-over seams in tubes formed in cross-roll piercing mills



Feb. 5, 1957 J. HOLMQUIST 2,730,119

METHOD OF AVOIDING LAPPED-OVER SEAMS IN TUBES FORMED IN CROSS-ROLL PIERCING-MILLS Filed Aug. 19, 1954 9 BY L l $7M @M W, 4%,! MM

METHOD OF AVOIDING LAPPED-OVER SEAMS IN IgBESFORMED IN CROSS-ROLL PIERCING John L. Holmquist, Aliquippa, Pa., assignor to The Natronal Supply fiompany, Pittsburgh, Pa., a corporation of Pennsylvania Application August 19, 1954, Serial No. 450,855

Claims. (Cl. 80-62) This invention relates to the piercing of heated metal billets by cross-rolling mills that comprise a piercing head held on the end of a mandrel bar in a piercing pass formed by variously shaped driven rolls whose axes are inclined to each other and to the pass line.

In thus forming relatively thick-wall tubular blanks for subsequent elongation and reduction in wall thickness to make seamless tubing, the prevailing practice,--and the only actual practice known to me, is to mount a piercing head on the forward end of a rotatable mandrel bar so that during a piercing operation the head is free to rotate as an idle roll while the billet is rotated by the action of the cross-rolls on its exterior surface.

Seamless tubes that have been formed from billets that contain radial cracks or seams, asthey not infrequently do, require subsequent grinding to improve their appearance. This is because inpresent cross-roll piercing operations, outside radial cracks and seams in billets are converted into conspicuous lapped-over seams on the outer surfaces of the tubes formed from such billets. If the seams are too deep and too much enlarged, the tubes are so defective that they cannot be used, but if they are not too deep, the unsightly seams can be ground away.

The object of the invention is to provide a method of piercing heated billets by the cross-rolling procedure, in which objectionable lapped-over seams at the periphery of. the resulting tubular blanks are avoided.

In the practice of this invention, such seams are avoided by modifying the normal deformation of the metal, this being done by positively rotating the piercing head on its longitudinal axis in thedirection of rotation of the billet with a torque that maintains outer radialseams in the billet substantially radial in thetubularblank formed from the billet.

The invention is illustrated in the accompanying drawings, in which Fig. 1 is a diagrammatic planview ofa cross-roll piercing mill equipped for the practice of this invention, the mill being shown in operation, and the piercing pass through the mill being shown in central horizontal longitudinal section;

Fig. 2 is an end view of a solid billet;

Fig. 3 is an end view of a tubular blank if pierced from the billet of Fig. 2 in accordance with the prior art;

Fig. 4 is an end view of a tubular blank if pierced from the billet of Fig. 2 in accordance with this invention;

Fig. 5 is a fragmentary end view of a tube wall containing a lapped-over seam; and

Fig. 6 is a similar view showing howthe seam would look if my method had'been used in forming the tube.

As shown in Fig. l, a cross-rolling mill comprises a pair of cross-rolls l and 2 whose axes are inclined to each other and to the line of pass between them, the rolls being suitably rotated at like peripheral speeds by shafts 3 and 4 that extend to a drive mechanism comprising motors and reduction gears which are generally indicated at 5 and 6. A mandrel bar 7, having its forward end provided with a piercing head8, is held against 2,780,119 PatentedF-eb. 5, 1957 longitudinal movement during each piercing operation, the bar bing so positioned that it holds the head in the throat of the pass.

In a piercing operation such as here illustrated, a heated solid billet 9 of cylindrical shape is moved into the bite of rolls 1 and 2, which engage its periphery and rotate it on its axis in a helical path toward the rear end of the mandrel bar. When the leading end of the billet strikes the forward end of the piercing head, the billet is centrally pierced by the head to form a central cavity in it, and the metal of the billet is so caused to flow around the mandrel that it emerges fromthe rolls in the form of a tube surrounding the mandrel bar. In some cases, particularly when large diameter billets are used for the production of large diameterseamless tubing, billets are given two cross-roll piercing passes, the workpiece or billet for the second pass being the tubular blank resulting from the first pass. This invention is applicable to the second as well as tothe first of such piercing operations.

During piercing operations as heretofore practiced, head 8 would be rotated on its axis only by reasonof its periphery being engaged by the inner wall of the cavity that it is forming in the billet. The billet is rotated on its axis by the action of driven cross-rolls 1 and 2 which engage the outer surface of the workpiece. In such prior practice the piercing head acts as an idle roll between the driven cross-rolls. The billets that are used in piercing mills often have radial cracks or seams in their peripheral regions, and the prevailing practice of piercing the billets converts such radial scams or cracks into unsightly laps, as indicated in Fig. 5. These lapped-over seams are so objectionable in appearance, even when the seams are not deep enough to require rejection of the pipe, that it is common practice to grind the outside of the pipe to remove them. Of course, the grinding adds to the cost of the pipe.

I-have found that by positively driving the mandrel head i; in the direction that the billet is rotated by rollsl and 2, lapped-over seams can be substantially eliminated, or made so unnoticeable as not to be objectionable. Accordingly, piercing head 8 ismounted on the front end of mandrel bar 7 for rotation by it, and the rear end of the bar is rotatably borne by a thrust block 13 which is provided with retracting mechanism to move the mandrel rearwardly at the completion of a piercing operation. This mechanism may comprise a piston rod14 whose forward end is attached to thrust block l3, its rear end being attached to a piston in cylinder 16 of a suitable fluid pressure mechanism. At the completion of a piercing operation, a pierced tubular blank may be stripped from mandrel bar 7 by actuating the retracting mechanism which moves thrust block 13 to the right. The end of the pierced blank then abuts against a stripper" block 17 until the mandrel bar is drawn through it, which permits the tubular blank to fall on a suitable conveyor. During a piercing operation, thrust block 13 is held rigidly in desired position by fingers 18 that are borne by a rock shaft'19 which may be turned to lift the fingers from the thrust block after the completion of a piercing operation.

Mandrel bar 7 is preferably rotated by an adjustable speed electric motor 21, having a shaft 22 equipped with a pinion 23 that meshes with an idle gear 24, which in turn meshes with a ring gear 25. This ring gear is mounted on a rotatable clutch drum 26 adapted to be engaged internally by a clutch (not shown) that is rotatably supported by thrust block 13 between the latter and the mandrel bar. When the clutch is released from the drum and latches 18 have been turned upward to release thrust block 13, the retracting mechanism may be actuated to pull back the thrust blocks, clutch and mandrel bar 7. In such withdrawing of mandrel bar7'from a pierced workpiece, drive motor 21, pinion 23, idle gear 24, ring gear 25 and clutch drum 26 remain stationary so that the inertia thereof does not have to be overcome by the retracting mechanism. The inertia of this driving mecha nism is of a substantial character, because it requires a relatively large motor to drive piercing head 8 in the practice of this invention.

The amount of driving torque exerted on the mandrel, and by it applied to the wall of the billet cavity containing the piercing head, is important. This was discovered in connection with my discovery of what causes lappedover external seams, which are seams that intersect the surface of the tubular blank at a large angle to the radius of the blank. Before the piercing operation those same seams extend substantially radially of the billet. The tilting or angling of the seams when the mandrel is not driven is due principally to the metal that engages the rolls traveling at a higher angular speed than the metal engaging the mandrel head, circumferential shear stresses being imparted to the wall of metal between the rolls and head. This ditference in angular speeds, i. e., revolutions per minute, results in the outer ends of the radial cracks or seams moving ahead faster than their inner ends, thereby causing the seams to spiral around the blank.

What takes place during piercing regarding external radial seams is illustrated in Figs. 2 to 6. An end of a cylindrical billet is represented in Fig. 2, on which a radial line 30 has been drawn, the outer end portion of which represents a radial seam. If the billet is pierced by the conventional method, it becomes the tubular blank shown in Fig. 3, but the radial line 3! now becomes line 31, which spirals around the tube so that the outer portion of the line, representing an outside seam, makes a large angle a with the radius of the tube. The rotation of the outer surface of the blank relative to its inner surface is indicated by the angle b, this rotation being accompanied by a shearing deformation of the metal between those surfaces. On the other hand, if the billet is pierced in accordance with my method, the line 30 can be made to take approximately the shape of the line 32 shown in Fig. 4, where it will be seen that the angle between the outer end portion of the line and the radius of the tube is quite small. That is, the outside seam that was radial in the billet shown in Fig. 2 remains nearly radial in the tubular blank. The same thing is not true of an inside seam, but such seams are more or less concealed and their appearance is not important. The particular lines 31 and 32 on Figs. 3 and 4 are correct for a given set of factors, including the contours of the crossrolls and piercing head, the diameters of the solid billet and tubular blank, and the inclination of the axes of the cross-rolls to the line of pass through the mill. It will be realized, however, that if any of these factors are changed, the shape of the lines may also change somewhat. Figs. 5 and 6 correspond to Figs. 3 and 4, respectivcly, but indicate how actual seams look under the old and new methods. The lapped-over seam 33 of the old practice, which may be open slightly, can be avoided by using my method, and then an outside seam 34 looks more like the unobjectionable one shown in Fig. 6.

To obtain the desirable result shown in'Figs. 4 and 6, the torque applied by the driven mandrel head to the wall of the cavity in the billet must act positively on the workpiece to impart rotation to it in cooperation with the cross-rolls. I have found that the torque should be great enough to rotate that wall at a greater angular speed than the outer surface of the workpiece in order to reduce circumferential shear in the outer portion of the blank as much as possible by reversing the direction of circumferential shear near the inner surface of the blank. This means that to apply the desired torque the mandrel head should be rotated faster by the mandrel bar than the head was rotated by the blank in the prior art. The best torque or speed for the head under any given conditions can be determined by experiment and observation of the tubular blanks, but in any case the torque applied to the wall of the cavity should be in a controlled amount stiflicient to maintain any outer radial seams in the billet substantially radial in the resulting blank so that objectionable lapped-over seams will be avoided.

Since the torque applied to the piercing head is through the mandrel bar, the latter is placed in torsion, the degree of which varies with the amount of torque transmitted to the head. The torsion in the bar twists it to a certain extent, and this twist must be built up in the bar in order that the head shall exert torque on the billet. As billet, piercing head and mandrel are all rotating in the same direction, it follows that momentarily, at the beginning of the piercing operation, the power driving the mandrel must be rotating it faster than the rotational speed of the billet. Then when the mandrel head comes in contact with the billet, the resistance of the head to slipping on the billet retards the rotation of the head, causing twist to develop in the mandrel bar. When the amount of twist that accumulates in the bar corresponds to the desired amount of torque, the rotational speed of the bar must decline so that no further twist will be accumulated. If a shunt wound direct current motor is used for driving the mandrel, the armature current can be indicated by an ammeter 36 in the armature circuit. The torque on the mandrel head is proportional to the armature current, so that current can be controlled manually or automatically to give the correct reading on the ammeter for the desired torque.

The horsepower of the mandrel-driving motor 21 is substantial, but it need not exceed one-half of the horse power required to drive the cross-rolls of a mill in which the piercing head acts as an idle roll. The power input for driving the piercing head reduces approximately an equal amount from the power required to drive the crossrolls when the piercing head acts as an idle roll, in view of which the total power required for the operation of a mill in the practice of this invention does not differ materially from that required for the operation of a piercing mill where the piercing head is not driven.

According to the provisions of the patent statutes, I have explained the principle of my invention and have illustrated and described what I now consider to represent 1ts best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. In a piercing between driven cross-rolls of a rotating heated metal billet provided with an axial cavity between the rolls, the method of avoiding objectionable lapped-over seams at the periphery of the resulting tubular pierced blank, comprising continuously applying torque to the wall of said cavity in the direction of rotation of the billet in a controlled amount sufficient to maintain any outer radial seams in the billet substantially radial in said blank.

2. In the piercing between driven cross-rolls of a rotating heated metal billet provided with an axial cavity between the rolls, the method of avoiding objectionable lapped-over seams at the periphery of the resulting tubular pierced blank, comprising continuously applying a predetermined torque in the direction of rotation of the billet to the wall areas of said cavity beside the crossrolls, said torque being sufficient to keep any outer radial seams in the billet from spiraling appreciably in said blank.

3. In the piercing between driven cross-rolls of a rotating heated metal billet provided with an axial cavity between the rolls, the method of avoiding objectionable lapped-over means at the periphery of the resulting tubular pierced blank, comprising continuously applying enough torque to the wall of said cavity in the direction of rotation of the billet to cause said wall to rotate at more revolutions per minute than the periphery of the pierced portion of the billet, whereby to maintain any outer radial seams in the billet substantially radial in said blank.

4. In the piercing of a rotating heated metal billet by a piercing head supported between driven cross-rolls by a long mandrel having a driving connection with the head, the method of avoiding objectionable lapped-over seams at the periphery of the resulting tubular pierced blank, comprising driving the mandrel faster than the billet is being rotated by the rolls before the billet engages the piercing head, feeding the rotating billet onto said head so that the head is slowed down and the mandrel twisted while torque on the head builds up, and then as soon as the desired amount of torque is reached reducing the speed of the mandrel to a speed that will maintain said desired torque, said desired torque having a value sufficient to maintain any outer radial seams in the billet substantially radial in said blank.

5. In the piercing method recited in claim 4, said lastntentioned speed being enough greater than the peripheral speed of the blank to rotate the head-engaging Wall of the blank forward relative to the outer surface of the blank.

References Cited in the file of this patent UNITED STATES PATENTS 361,957 Mannesmann .d Apr. 26, 1887 389,585 Mannesmann Sept. 18, 1888 971,512 Adams Oct. 4, 1910 2,048,925 Diescher July 28, 1936 

